Hot-Dip Galvanized Sheet Steel with Excellent Press Formability by fdh56iuoui



                   Hot-Dip Galvanized Sheet Steel
        with Excellent Press Formability and Surface Quality
                     for the Automotive Panels*

                                                                    A new type of a hot-dip galvanized steel sheet (GI
                                                                 steel sheet), excellent in press formability, weldability
                                                                 and surface appearance, for automotive bodies has been
                                                                 developed. As for press formability, through the opti-
                                                                 mization of surface roughness effected by texture control
                                                                 with skin-pass rolling and by the use of a high-lubricat-
                                                                 ing oil, sliding characteristics has been improved. Con-
  Kazuhiro Abotani     Kazuhiro Hirohata       Tetsuya Kiyasu
Staff Assistant       General Manager,      Staff Manager,
                                                                 cerning an advantage in weldability, with the suitable
Manager,              Cold Rolling Dept.,   Products Service &   selection of coating and substrate chemistry, the life of
Cold Rolling Tech-    Mizushima Works       Development Sec.,    welding electrode has been extended. Improved surface
noloty Sec.,                                Products Service &   quality for the better exterior appearance was achieved
Cold Rolling Dept.,                         Development Dept.,
Mizushima Works                             Mizushima Works      through the introduction of such measures as the pre-
                                                                 vention of bath wrinkle patterns by adjusting a wiping
                                                                 condition, the avoidance of dross adherence with the use
                                                                 of bath exclusive for GI use, and the substantial reduc-
                                                                 tion of spangles by using cooling rate control after
                                                                 galvanizing. The hot-dip galvanized steel sheet demon-
1 Introduction                                                   strates distinguished characteristics as a material sheet
   In response to the problem of auto body corrosion,            for automotive outer panels. The newly developed GI
sheet steel for automotive body panels has undergone a           sheets are put into commercial production,
great change from the conventional cold rolled sheet
steel to coated steel sheets. As a countermeasure against
corrosion caused by road salt in North America and
other countries of cold climates, various types of coated
sheet steels were developed and began to be used in the          ica is electrogalvanized sheets (EG) with a compara-
second half of the 1970s. Therefore, the percentage of           tively heavy galvanized coating. In contrast to this, many
coated products increased, reaching more than 2/3 of all         auto makers in Japan use galvannealed steel sheets
automotive panels in the 1990s.                                  (GA), which have an alloyed coating, in preference to
   Coated sheet steels which are used in automobiles             EG. In Europe, in addition to EG or GA, auto makers
must not only provide excellent corrosion resistance, but        mainly use hot-dip galvanized sheets (GI), which are
must also satisfy a variety of other performance require-        produced by hot-dip galvanizing in the same manner as
ments, including press formability, weldability, phos-           GA sheets, but which are not given subsequent treatment
phatability and paintability. In actual production, the          to alloy the metal coating and steel substrate. Moreover,
priorities and concepts which are adopted in corrosion           recently, there has been a continuing increase in makers
prevention countermeasures differ among auto makers              who use GI sheets not only in inner panels but also in
and various types of corrosion-resistant sheet steels are        exterior panels, resulting in increased demand for this
applied as required, by taking advantage of the features         material.1)
of the respective materials.                                        Historically, GI steel sheets have been used mainly as
   For example, the mainstream product in North Amer-            a construction material. In comparison with electrogal-
6666   6                                                         vanized products, GI has the advantages of lower pro-
* Originally published in Kawasaki Steel Giho, 34(2002)2, 66–    duction costs and easy manufacture of products with a
  70                                                             heavy coating weight. However, a number of problems
remained to be solved with GI, including relatively poor                                                                              0.8

surface quality and inadequate performance in the prop-

                                                                                        Thickness reduction of steel with painting
erties which are necessary for manufacturing and assem-                                                                                                                                         CR
bling auto bodies, such as press formability and spot

                                                                                                   (CCT: 80 d) (mm)
weldability, and improvements were needed.                                                                                            0.5
   This report describes the quality criteria of a GI sheet                                                                                           Zn-Ni20
                                                                                                                                      0.4                                             EG20
steel which solves the previously-mentioned problems
and has been applied successfully to automotive outer                                                                                 0.3
panels, together with a stable production technology for
this GI product.

2 Properties of Hot-Dip Galvanized Steel                                                                                                0
                                                                                                                                            0    0.1     0.2    0.3     0.4     0.5     0.6     0.7   0.8
  Sheets (GI)                                                                                                                                          Thickness reduction of steel without
                                                                                                                                                           painting (SST: 720 h) (mm)
  2.1 Corrosion Resistance
                                                                                     Fig. 2 Perforation corrosion resistances of pre-
   Figure 1 shows the results of a corrosion test of vari-                                  coated steel sheets with and without painting
ous types of Zn coated steel sheets.2) In this test, which
was conducted at seashore test site in Okinawa (approx-                             nized coating layer has poor sliding properties with the
imately 10 m from the coastline), various types of coated                           press die, and this can cause cracks and other problems.
steel sheets with different Zn coating weights were                                 As a result, there are cases in which it is impossible to
exposed for four years in the as-galvanized condition.                              take full advantage of the excellent mechanical proper-
Corrosion resistance was improved as the coating weight                             ties of the base material in the press forming process.
increased. Because a heavy zinc coating can easily be                                  Figure 3 shows the friction coefficient in the case of
realized at low cost with GI sheets, high rust resistance                           plane sliding (flat type die) at a sliding speed of
can be obtained. In particular, GI demonstrates a rust-                             20 mm/min for specimens of GA, EG, cold rolled, and
resistance effect in the hem and flange parts of the auto                            GI sheets with a washing oil coating weight of 1.5 g/m2.
body, where paint coating cannot be expected to guaran-                             The die blocks were of SKD1 material, with dimensions
tee rust prevention.                                                                of 20 mm in the sliding direction and 20 mm in the
   Figure 2 shows the results of an evaluation of the cor-                          width direction, and were pressed against the surface of
rosion resistance of the various types of sheet steels with                         the steel sheets with a pressing force of 1 960 N. In com-
and without painting in cyclical corrosion test (CCT).                              parison with the GA, EG, and cold rolled steel sheets it
Zn-Ni alloy coated sheets showed relatively high perfo-                             was found that the GI sheet had a superior sliding prop-
ration corrosion resistance without painting, and GA                                erty, as seen in the smaller coefficient of friction.
sheets tended to exhibit excellent corrosion resistance                                An investigation was carried out to determine the
when painted. It was found that GI sheets have excellent                            effect of the surface roughness, anti-rust oil used, and
corrosion resistance after painting, even against perfora-                          pressing force during the test on the friction coefficient
tion corrosion.2)                                                                   of GI sheets. High roughness and low roughness materi-
  2.2 Press Formability
                                                                                                                                                                              1 960 N
   Even if a base steel sheet possesses satisfactory                                                                                                                                          20 mm/min
mechanical properties, the sheet metal flow will be lim-                                                                              0.12

ited in the part where the bead must pass if the galva-

                                                                                                   Friction coefficient

                                                  Zn-Ni          Without paint                                                       0.08
          Corrosion depth (mm)

                                 0.12              EG                                                                                0.06
                                                          GA                                                                         0.04
                                 0.04                                     GI
                                                        GA        GA                                                                 0.02
                                        0   10   20 30 40       50 60    70    80                                                      0
                                                 Coating weight (g/m2)                                                                           GA             EG             CR              GI

 Fig. 1 Relationship between coating weight and                                      Fig. 3 Influence of coating type on friction coeffi-
        corrosion depth examined in various Zn                                              cient (Washing oil: 1 g/m2, Oil viscosity
        coated steel sheets exposed in Okinawa                                              17 mm2/s, Sliding speed: 20 mm/min, Press-
        seashore for 4y                                                                     ing force: 9.8 MPa)
18                                                                                                                                              KAWASAKI STEEL TECHNICAL REPORT
als were manufactured by changing the roughness of the                                                                                          0.18
work rolls used in temper rolling after galvanizing.

                                                                                                                         Friction coefficient
When a low pressing force was applied, the friction                                                                                             0.12
coefficient of the low roughness sheet was lower. How-                                                                                            0.1
ever, it was found that the effect of roughness on the                                                                                          0.08
friction coefficient decreased as the pressing force                                                                                             0.06
increased, and there was no difference between the high                                                                                         0.04
roughness and low roughness materials when the press-
ing force exceeded 6 860 N (Fig. 4).                                                                                                                     Oil A          Oil B         Oil C
    Figures 5 and 6 show the friction coefficient in plane                                                                 A: Viscosity 17 mm2/s (Standard anti-rust oil)
                                                                                                                          B: Viscosity 36 mm2/s (Lubrication type anti-rust oil)
sliding (flat type die) and columnar sliding (roll type                                                                    C: Viscosity 26 mm2/s (Lubrication type anti-rust oil)
die) when three types of anti-rust oil were applied to GI
sheets at a coating weight of 1.5 g/m2. Here, the die                                      Fig. 5 Influence of corrosion lubricating oil on GI
blocks were of SKD11 material, with dimensions of                                                 friction coefficient (Flat type die)
50 mm in the sliding direction and 20 mm in the width
direction, but as in the previous test, the pressing force                                                              0.12                                                1 960 N
                                                                                                                                                                                           20 mm/min
was 1 960 N and the sliding speed was 20 mm/min.                                                                        0.11
    A comparison of two high lubrication type anti-rust

                                                                                                 Friction coefficient
oils (B: Kinematic viscosity, 36 mm2/s, C: Kinematic
viscosity, 26 mm2/s) with a standard type anti-rust oil                                                                 0.09
(A: Kinematic viscosity, 17 mm2/s), the friction coeffi-                                                                 0.08                           Oil A
cient was reduced in both the flat die sliding test and the                                                                                             Oil B
                                                                                                                        0.07                           Oil C
roll die sliding test by using a high lubrication oil. Thus,
it is possible to obtain satisfactory press formability with                                                            0.06
                                                                                                                           1 000                       2 000     3 000    4 000   5 000       6 000
GI sheets by imparting the appropriate roughness to the                                                                                                        Pressing force (N)
material and applying a high lubrication type anti-rust                                                                               A: Viscosity 17 mm2/s (Standard anti-rust oil)
oil.                                                                                                                                  B: Viscosity 36 mm2/s (Lubrication type anti-rust oil)
                                                                                                                                      C: Viscosity 26 mm2/s (Lubrication type anti-rust oil)
  2.3 Weldability
                                                                                           Fig. 6 Influence of pressing force lubricating oil on
   Auto bodies are assembled by performing spot weld-                                             GI friction coefficient (3 roll type die)
ing at 3 000 to 4 000 spots per automobile. In compari-
son with cold rolled sheets, the number of continuous                                        Spot weldability (number of continuous spot welds)
spot welds with the same electrode is lower with Zn                                       can be improved by adding the element boron (B) to the
coated sheets. The reason for this is considered to                                       substrate steel sheet. It has been reported that the fatigue
include the following two points, which are due to the                                    strength of spot welds can be improved by this measure,
presence of a soft, low melting-point Zn coating layer:                                   which suppresses grain growth in the heat affected zone
(1) A high current welding is necessary in order to                                       (HAZ). However, preventing softening of the steel sheet
   expand the route of current passage.                                                   by suppressing grain growth is also considered to be an
(2) As a result, the temperature of the electrode tip is                                  effective measure for reducing electrode wear. As a
   high and the diameter of the worn electrode tip tends                                  result of various attempts to increase spot weldability by
   to increase.                                                                           improving the characteristics of the steel sheet itself, as
                                                                                          mentioned briefly here, it was possible to extend elec-
                                                                                          trode life substantially, as shown in Fig. 7.
                              0.09                               Ra    1.61 µm              2.4 Phosphating Property
                                                                 Ra    0.88 µm
                                                                                             Phosphating is performed as a pretreatment before
       Friction coefficient

                                                                                          painting steel sheets, and has a tendency to improve cor-
                              0.07                                                        rosion resistance after painting and improve adhesion
                                                                                          between the paint and the coating film or steel sheet.
                                                                                          Phosphating characteristics are evaluated by observing
                                                                                          the crystal structure of the phosphate coating after ordi-
                                                                                          nary phosphating.
                              0.04                                                           Photo 1 shows the appearance of crystals observed by
                                     0   2 000    4 000     6 000     8 000      10 000   SEM when cold rolled steel sheets, GI sheets, and GA
                                                 Pressing force (N)
                                                                                          sheets were immersed in a phosphating solution for 2
 Fig. 4 Influence of roughness on GI friction coeffi-                                       min, washed in water, and then dried, using a PB-WL 35
        cient                                                                             manufactured by Nihon Parkerizing Co., Ltd. With the
No. 48 March 2003                                                                                                                                                                                      19
                       8.00                                                                 cold rolled sheets, grains having a phosphophyllite crys-
                                                              Standard type                 tals were seen, and there were no significant differences
                                                              After improvement             between the two types of sheet in either the size or shape
Button diameter (mm)

                                                                                            of the crystals.
                                                                                               With GA steel sheets, it has been reported that a paint
                                                                                            coating defect called “crater” tends to occur in electro
                                                                                            deposition coating.3,4) Therefore, Fe-P coating is per-
                       2.00                                                                 formed as a top layer in order to prevent craters. In com-
                                                                                            parison with GA sheets, craters do not occur with GI
                                                                                            sheets, which show satisfactory electro deposition coat-
                              0      400           800          1 200       1 600   2 000   ing performance.2)
                                            Number of continuous spot welds

        Fig. 7 Comparison between electrode life of GI                                      3 Production Technology for Hot-Dip
               before and after improvement                                                   Galvanized Steel Sheets (GI)
                                                                                              3.1 Improvement of Surface Quality
                                                                                                 3.1.1 Wrinkle pattern
                                                                                                 When steel sheets are coated in a Zn bath, the Zn
                                                                                            coating weight is controlled by an air or nitrogen wiping
                                                                                            gas jet above the Zn pot. In this process, however, the
                                                                                            wiping gas jet causes the steel strip to vibrate, and the
                                                                                            flow of molten Zn in the coating layer is irregular. As a
                                                                                            result, a wave-shaped flow pattern called a “wrinkle”
                                                                                            pattern frequently occurs, as illustrated in Photo 2. In
                                                                                            particular, this wrinkle pattern must be prevented in
                                           (a) Cold-rolled steel sheet                      materials which are to be applied to the outer panels of
                                                                                            automobiles because it is detrimental to the surface
                                                                                            properties of the paint film, and especially to smooth-
                                                                                            ness, when the Zn-coated surface is the substrate surface
                                                                                            for painting.
                                                                                               For this reason, a wrinkle pattern prevention technol-
                                                                                            ogy was established at the continuous hot-dip galvaniz-
                                                                                            ing line (CGL) at Mizushima Works by the control of
                                                                                            wiping conditions (wiping pressure, gap between nozzle
                                                                                            and steel strip, height of nozzle above Zn bath surface).
                                                                                            Photo 3 shows the surface property of a GI sheet after
                                                                                            establishment of this technology.
                                                     (b) GI                                      3.1.2 Countermeasures against spangle
                                                                                                 When the zinc coating on a steel sheet solidifies,
                                                                                            dendrite-type crystals form and grow around a core of
                                                                                            solidified Zn. In some cases, this results in a flower-
                                                                                            shaped pattern called “spangle” on the surface of the

                                                    (c) GA

        Photo 1                   Scanning electron micrographs of phos-                                                         10 mm
                                  phate coating formed on (a) CR (b) GI and
                                  (c) GA                                                            Photo 2   Wrinkle pattern of GI surface
20                                                                                                        KAWASAKI STEEL TECHNICAL REPORT
                                                                                                       10 mm
                                     10 mm

                                                               Photo 4        Spangle pattern of conventional GI (after
 Photo 3    GI surface after improvement on the wip-
                                                                              etching with CuSO4 aqueous solution)
            ing conditions

galvanized sheet. Depending on the use, this flower pat-
tern is considered desirable in some applications, such
as building materials, where it contributes to the beauty
of the material surface. Conversely, however, when a
sheet is to be used as an automotive panel, a minimum
spangle material with fine sized spangles is necessary
because the surface irregularities and differences in
crystal orientation mar the appearance after painting. It                                             10 mm
is known that the size of spangles is influenced by the Pb
and Sb contents of the Zn bath and by the solidification            Photo 5        Spangle pattern after improvement
rate of the coated layer. The effect of the Pb and Sb con-
tents is explained as follows: Pb and Sb melt in molten                                    Dross (Fe2Al5)
Zn but have low solubility in solid Zn. For this reason,
Pb and Sb are discharged from the Zn layer as it solidi-                 Zn
fies after coating, and this reduces the solidification
point of the unsoldified parts. The solidification rate
                                                                                              Strip (Fe)
influences the spangle morphology because the growth                                                            20 µm
rate of solidified crystal cores increases at higher solidi-
fication rates.                                                 Photo 6        Cross sectional micrographs of dross on GI
   Photo 4 shows the appearance of spangles on a con-
ventional sheet; the photograph was taken after the sheet     amount of floating dross is formed by the reaction
was etched by using a CuSO4 aqueous solution.                 shown below.
   With the new CGL at Mizushima Works, it is possible
                                                                      2FeZn7 (Bottom dross) 5Al
to manufacture steel sheets with a fine spangle size, as
                                                                                     Fe2Al5 (Floating dross)           14Zn
shown in Photo 5, by minimizing the Pb content of the
Zn coating bath and controlling the cooling rate above           This floating dross adheres to the sheet surface and
the Zn pot.                                                   causes surface defects, as shown in Photo 6.
                                                                 At CGL of Mizushima Works, one additional Zn pot
     3.1.3 Dross adherence
                                                              was installed in November 2001, making it possible to
      Another quality problem with hot-dip galvanized         manufacture products using exclusive pots for the GA
sheets is dross adhering to the steel sheet. Dross is a       bath and GI bath. This eliminated the need for bath
problem because it crystallizes out of the solution in the    switching, enabling production under stable bath condi-
Zn pot and causes a surface defect called “pimple” after      tions. As a result, the amount of generated dross is
press forming. Dross is formed by reactions between Fe,       reduced to the minimum possible level, and the number
which is eluted from the steel strip into Zn coating bath,    of dross particles adhering to the steel strip can be
and the added Al and molten Zn in the bath, and is cat-       greatly reduced, as shown in Fig. 8. This has made it
egorized as either or an Fe-Al and an Fe-Zn intermetal-       possible to produce GI sheets with satisfactory surface
lic compound. The Fe-Al dross (floating dross) floats in        quality.
the pot, while the Fe-Zn dross is found at the bottom of
                                                                3.2 Texturing Technology (Roughness Control)
the pot (bottom dross). It is known that the formation of
both compounds is strongly affected by the Al concen-           As mentioned in the previous chapter, the roughness
tration in the bath and the bath temperature.5–7)             of the sheet has a large influence on press formability.
   When GI and GA are produced by using one pot, the          Roughness can be controlled mainly by changing the
Al concentration in the bath is increased greatly when        work roll roughness during skinpass rolling, and by
the GA bath is switched to the GI bath, and a large           changing the skinpass rolling load and tension. As roll
No. 48 March 2003                                                                                                        21
                                                                  Dross defect appears in appearance
                                 4.5                              after painting                       devices. EDT provides a wide texturing range for multi-
                                                                                                       ple indexes (Ra, PPI, Wca) and enables texturing with
       Number of dross (cm 2 )   3.5                                                                   reduced deviation.
                                                                                GA and GI Zinc pot
                                  3                                             Only GI Zinc pot
                                                                                                          Figure 10 shows the sheet surface profile of GI sheets
                                 2.5                                                                   which were rolled with shot blasted and EDT textured
                                  2                                                                    rolls. By using the EDT texturing equipment, it is possi-
                                 1.5                                                                   ble to manufacture GI with excellent press formability
                                  1                                                                    by producing sheets with the proper roughness depend-
                                 0.5                                                                   ing on the press forming application.
                                       0            100        200       300        400        500
                                                              Dross size (µm)
                                                                                                       4 Conclusion
                                 Fig. 8 Number of dross on GI sheet                                       When hot-dip galvanized sheet steel (GI) for automo-
                                                                                                       tive applications is to be used in automotive panels, var-
                                                                                                       ious problems arise, including the problems of press
                                                                                                       formability, spot weldability, and surface quality.
                                                                            Shot blast
                                                                                                       Kawasaki Steel has improved the performance of GI
                                                                                                       sheets in each of these areas, as summarized below. As a

                                              300                                                      result, it is now possible to produce GI sheets with
                                                                                                       excellent press formability and surface quality for use in
                                                                                                       auto body outer panels.
                                              100                                                      (1) Features of Hot-Dip Galvanized Sheets
                                                                                                          (a) In addition to the good mechanical properties of
                                                    0     1   2    3    4  5         6     7                 the steel sheet, which had been established previ-
                                                                   Ra (µm)
                                                                                                             ously, the developed GI sheets also show satisfac-
 Fig. 9 Influence of skin-pass roll texturing technol-                                                        tory press formability as a result of improvement in
        ogy on PPI and roughness                                                                             sliding properties by roughness control and use of a
                                                                                                             high lubrication type anti-rust oil.
                                                                                                          (b) The GI sheets show satisfactory continuous spot
                                                                                                             weldability as a result of improvement in the coat-
                                                                                                             ing layer and substrate steel chemistry.
                                                                                                       (2) Production Method for Hot-Dip Galvanized Sheets
                                                                                                          (a) It was possible to prevent the “wrinkle” defect by
                                                                                                             optimizing wiping conditions.
                                                                                                          (b) It has become possible to produce sheets with a
                                                                                                             fine spangle size by optimizing the coating bath
                                                                                                             composition and cooling rate after coating.
                                             (a) Shot blast            Ra    1.10 µm, PPI        146      (c) It has become possible to produce sheets with
                                                                                                             minimal dross adherence by stabilizing bath condi-
                                                                                                             tions, which was achieved by adopting an exclu-
                                                                                                             sive-use pot for GI products.
                                                                                                          (d) Control of the roughness profile of the skinpass
                                                                                                             work rolls by using an electro discharge texturing
                                                                                                             (EDT) device has made it possible to transfer the
                                                                                                             proper roughness to the steel sheet, as required by
                                                                                                             the press application.

                                             (b) EDT                   Ra    1.18 µm, PPI        186
 Fig. 10                           Influence of skin-pass roll texturing on tex-                        1) D. Quantin and F. Ronin: 40th MWSP Conf. Proc., ISS, (1998)
                                   ture and roughness for GI sheet                                     2) T. Kubota and M. Yamashita: Tetsu-to-Hagané, 7(1991), 1087
                                                                                                       3) N. Sato and S. Tanaka: Tetsu-to-Hagané, 8(1986), 1084
texturing equipment, cold strip mill at Mizushima Works                                                4) K. Yamato, T. Ichida, and T. Irie: Kawasaki Steel Giho, 21
                                                                                                          (1989)3, 222
introduced an electro discharge texturing (EDT) device
                                                                                                       5) H. Yamaguchi and T. Hisamatsu: Tetsu-to-Hagané, 60(1974),
in December 1999, in addition to the existing shot blast                                                  96
and laser roll texturing devices. Figure 9 shows the                                                   6) T. Ooi: CAMP-ISIJ, 5(1992), 1736
available texturing range with each of the roll texturing                                              7) H. Koumura: Tetsu-to-Hagané, 81(1995), 43

22                                                                                                                   KAWASAKI STEEL TECHNICAL REPORT

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